Where did the term "proof" come from?

Hello fellow talkbassers,
Come this wednsday I will be 21 years old. I'm really excited. Now I have a question for you guys and gals. Where did the term "proof" originate from? Why don't we just say 40% alcohol instead of 80 proof. I think saying 40% alcohol is clearer. Thanks guys and gals, this has been puzzling me for the longest time.
Sincerely,
-Greg P

Proof is another (older) measure of the strength of an alcoholic liquid.
It had its origins in days when a simple test was needed that the liquor did indeed contain a *correct* measure (or more) of alcohol. And it was indeed a simple test.
Some of the liquor was poured over a little gunpowder and ignited. If the alcohol content was adequate, then it would burn 'just right' with a steady blue flame and eventually ignite the gunpowder. If there was insufficient alcohol then it would fizzle out and the gunpowder would be too wet to burn. The 'just right' condition 'proved' the liquor and it was declared to be '100% proof'.
This simple test was clearly cumbersome to perform and was later replaced by using a specially graduated hydrometer to measure the specific gravity. This was far more objective and allowed precise statements to be made as to how much different it was from being 100% proof. This gave rise to "under-proof" and "over-proof" measures.

In the US a very simple relationship is defined between 'proof' and %AbV. It is that
proof = 2 × %AbV
So, 180 proof (US) = 90 %AbV.

It the UK it was laid down by an Act of Parliament in 1816 that "a quantity of 100% proof liquor would have the same volume as 12/13 ths of the same volume of pure water. (That is twelve thirteenths)
So,
100 proof (UK) = 57.06 %AbV
200 proof (US) = 100 %AbV = 175.25 proof (UK)
100 proof (US) = 50 %AbV = 87.6 proof (UK)
These conversions are handled in the
AbV and Equivalent Values
Calculator